A battery having a completely sealed structure, which provides enhanced sealing and excellent storage. But, where it is heated to high temperatures, used in an abnormal manner such as charging at a high voltage and large current, or short-circuited internally so that the internal temperature becomes abnormally high, the internal pressure of the battery increases. As a result, the battery can explode and hence the contents of the battery will scatter. This may cause damage to people and instruments in the vicinity of the battery.
For this reason, in order to prevent the pressure within the battery from exceeding the pressure at which the battery explodes, a proposal for a sealed type battery was made in which a cut or check is previously made and the pressure is released through its tearing (e.g. Examined Japanese Utility Model Publication No. Sho 58-17332 (1983), Unexamined Japanese Utility Model Publication No. Sho 60-65970 (1985)).
In the conventional sealed type battery in which a cut is made at a portion of the battery can, it is difficult to control precisely a variation in the remaining thickness of the cut in a machining such as stamping or wet etching. Thus, although the above conventional sealed type battery can improve the safety for explosion, when external shock is applied to the battery can, the sealing state of the battery can is likely to be broken. For this reason, a safety valve device was demanded which can operate stably and precisely at a desired pressure within a pressure range where safety is assured.
Accordingly, for the sealed type battery, in order to provide a safety valve device which operates stably and precisely at a desired pressure in accordance with an increase in the internal pressure, there was proposed. An explosion proof safety valve for the sealed type battery in which a check is formed at a portion of a battery can characterized in that at least a portion of the battery can has a multi-layer structure composed of two or more metal plates bonded together and at least one of the metal plates is made of a metal plate having a single or plural holes or through-groves (Unexamined Japanese Patent Publication No. Hei 5-314959 (1993)).
The above proposal intends to control the remaining thickness of the cut by the thickness of the metal plates bonded together so that the processing accuracy of the remaining thickness of the cut is enhanced and a variation in the operating pressure of the explosion-proof mechanism for a battery can is removed.
As described above, it was very efficient to provide a safety valve device for the battery cell so as to improve the safety performance of the sealed type battery. But, it is also very important to apply the technique of effectively preventing the internal short-circuiting of the battery, which may be a cause of battery explosion, to the internal structure of the battery.
Specifically, as one of the conventional sealed type batteries, there was proposed a sealed type battery having a specific structure in which a group of spiral electrodes composed of band-like positive electrodes and negative electrodes wound in an elliptical spiral shape through a separator are inserted into the battery can from the square or elliptical opening face of a tubular case in its section in such a manner that the winding axis of the group of spiral electrodes is in parallel to the opening face of the battery can and thereafter the opening face is hermetically sealed. Such a battery has a problem of mechanical short-circuiting caused when the positive electrodes and negative electrodes winding-slip are brought into direct contact with each other because of vibration shock or the like, or indirectly short-circuited with each other through the battery can.
Further, the above sealed type battery having the group of spiral electrodes described above in which the positive electrode is made of lithium composite oxide and the negative electrode is made of lithium intercalation compound has a problem of chemical short-circuiting wherein lithium ions deintercalated from the positive electrode on the inside of the winding-back portion of the group of spiral electrodes are deposited in dendrite on the negative electrode and penetrate through the separator thereby to produce short-circuiting. This is due to the fact that on the inside of the winding-back portion in the neighborhood of the winding core of the elliptical electrodes with a small curvature of the winding-back portion, the area of the negative electrode plate opposite to the positive electrode plate is relatively small and the amount of lithium absorbed in the negative electrode is less than that of the amount of lithium discharged from the positive electrode.
The safety valve device proposed in Unexamined Japanese Patent Publication No. Hei 5-314959 has a multi-layer structure composed of two or more metal plates pasted together, and thermal crimping is recommended as bonding means.
However, bonding the metal plates together makes it difficult to assure sufficient fixing strength. Such a means is insufficient in view of the reliability for airtightness.
Further, bonding by thermal crimping, which gives thermal influence on the environment, is not preferable. For example, thermal crimping may cause components made of resin attached to a cover plate on the inside of the battery to chemically transform.
In a safety valve mechanism for the battery having a multi-layer structure composed of stacked metal plates having a hole or through-groove and no such a hole or through-groove, the problem to be solved is to fix the metal plates to be stacked without bonding them together to assure sufficient fixing strength, thereby enhancing the reliability for air-tightness.
As a technique for preventing mechanical short-circuiting of a battery, a method has been proposed in which an insulating plate is inserted in a plane perpendicular to the winding direction of a group of electrode plates inside a case. This method gives rise to another problem that the number of components and/or working steps increases thereby increasing production cost.
In order to prevent chemical short-circuiting, a winding core is inserted in a group of spiral electrodes to increase the curvature of a winding-back portion so that the area of a negative electrode opposite to a positive electrode is maintained at a predetermined value or more thereby preventing the amount of lithium absorbed in the negative electrode from becoming smaller than that of a lithium discharged from the positive electrode. This method, however, has defects that inclusion of the winding core within the battery reduces the battery capacity and increases the weight of the battery.